Strain measurement is of particular importance to automotive vehicle designers. Conventional strain measurement is often conducted using an electrical strain gauge. Resistance strain gauges, extensometers, and capacitor strain gauges are examples of such conventional electrical gauges. In the design of automotive vehicles it is often necessary to measure hundreds of locations for strain for any given test. Electrical strain gauges require bonding and wiring which, in an automotive testing environment, is a time consuming set up process. Also, once an electrical strain gauge is used it must be discarded which can be very costly in automotive testing. Further, conventional strain gauges are inaccurate when exposed to high temperatures and high successive loading which is an undesirable testing limitation in automotive design.
Efforts have therefore advanced in the automotive strain measurement field to develop a noncontacting and nonconsumable method of measuring strain. One such method is known as shearography. According to this method, two laterally-displaced images of the object, which consist of random speckle patterns, are made to interfere to form a single speckle pattern. The pattern is random, and depends on the characteristics of the surface of the object. When the object is deformed, by temperature, pressure, or other means, the random interference pattern will change. The amount of the change depends on the soundness of the object. A comparison of the random speckle patterns for the deformed and undeformed states, which forms a fringe pattern, gives information about the structural integrity of the object. The method is called shearography because one image of the object is laterally-displaced, or sheared, relative to the other image.
Another noncontacting and nonconsumable strain measurement method, which was developed with the advent of the laser, is electronic speckle pattern interferometry (ESPI). In ESPI, a beam of laser light is directed onto the test object and reflected onto an image sensor. At the same time, a reference beam is also directed towards the sensor. The reference beam may be a "pure" beam or it may be reflected from a "reference" object. Both the object beam and the reference beam are nearly parallel when they reach the image sensor, so the spatial frequency of the interference speckle patterns is relatively low. Thus, the image sensor can be a video camera, or its equivalent.
There are many disadvantages associated with shearography and ESPI. ESPI requires an object beam and a reference beam of coherent light. The presence of two distinct beams increases the complexity of the optical system. The ratio of intensities of the object and reference beams must be carefully controlled, and the path lengths of the beams must be matched. Also, the use of lasers present safety issues as well as high cost. Both ESPI and shearography are full field strain measurement methods and require highly complex, and relatively inaccurate, computational methods to derive strain. Further, ESPI and shearography are highly sensitive to vibration. The slightest movement of either the object or the apparatus can ruin the pattern. Thus both methods require special vibration isolation precautions, and are not yet practical for strain measurement in an automotive vehicle testing environment. Still further, both methods require that the entire object surface be painted or processed for testing which adds cost to the process. Finally, ESPI and shearography methods create speckle noise which must be filtered by a noise reduction algorithm, further adding to the cumbersome nature of the processes.
Interferometric point wise, rather than full field, strain measurement is also an example of noncontacting strain measurement but is subject to the same shortcomings as ESPI. A problem associated with both full field and point wise noncontacting strain measurement, which is of great importance in automotive design and testing, is the method uninteruptibility. Put another way, once the particular apparatus is set up to measure strain it can not be removed in-between pre and post-loading. In automotive testing it is desired to take an initial, pre-load reading with the testing apparatus and then remove the apparatus for cycling. The automobile could, for example, be cycled for a predetermined period of time or distance with the apparatus being reapplied to the testing area for a post-load reading. This technique is impossible with the aforementioned noncontacting strain measurement methods.
Accordingly, it is seen that a need exists in the art for a method of using an automotive vehicle strain gauge which is noncontacting, has an uncomplicated strain measurement calculation, is not subject to the harsh vibratory environment of an automobile, is removable between the pre versus post loading phase, and is reusable, accurate, and easy to use.